How Does a Quantum Computer Work?
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How Does a Quantum Computer Work?


A classical computer performs operations using
classical bits, which can be either zero or one. Now in contrast, a quantum computer users
quantum bits or qubits. And they can be both zero and one at the same time. And it is this
that gives a quantum computer its superior computing power. There are a number of physical objects that
can be used as a qubit. A single photon, a nucleus or an electron.
I met up with researchers who were using the outermost electron in phosphorous as a qubit.
But how does that work? Well, all electrons have magnetic fields, so they are basically
like tiny bar magnets. And this property is called spin. If you place them in a magnetic
field they will align with that field, just like a compass needle lines up with the magnetic
field of the earth. Now this is the lowest energy state, so you
could call it the zero state or we call it for the electron, spin down. Now you can put
it in a one state, or spin up, but that takes some energy.>>If you took out the glass from your compass
you could turn the needle the other way, but you would have to apply some force to it.
You have to push it to flip to the other side. And that is the highest energy state. In principle,
if you were so delicate to really put it exactly against the magnetic field, it would stay
there.>>Now so far this is basically just like
a classical bit. It has got two states, spin up and spin down, which are like the classical
one and zero. But the funny thing about quantum objects is that thy can be in both states
at once. Now when you measure the spin it will be either up or down. But before you
measure it, the electron can exist in what is called a quantum super position, where
these coefficients indicate the relative probability of finding the electron in one state or the
other. Now it is hard to imagine how this enables
this incredible computing power of quantum computers without considering two interacting
quantum bits.>>Hello.
>>Hi. Now there are four possible states of these
two electrons.>>You could think that, well, that is just
like two bits of a classical computer, right? If you have two bits you can write zero, zero;
zero, one; one, zero; one, one. Right? There is four numbers. But these are still just two bits of information.
Right? All I need to say to determine which one of the four numbers you have in your computer
code is the value of the first bit and the value of the second bit. Here, instead, quantum
mechanics allows me to make super position of each one of these four states. So I can
write a quantum mechanical state, which is perfectly legitimate, that is some coefficient
times this plus some coefficient times that plus some coefficient times that plus some
coefficient times that. So determine the state of this two spin system,
I need to give you four numbers, four coefficients, whereas in the classical example of the two
bits, I only need to give you two bits. So this is how you understand why two qubits
actually contain four bits of information. I need to give you four numbers to tell you
the state of this system, whereas here I only need two. Now if we make three spins, we would have
eight different states and it could give you eight different numbers to define the state
of those three spins, whereas classical it is just three bits.
If you keep going, what you find is that the amount of equivalent classical information
contained by N qubits is two to the power N classical bits. And, of course, the power of exponentials
tells you that once you have, let’s say, 300 of those qubits in what we call the folient
angle state, so you must be able to create these really crazy states where there is a
super position of all three angles being one way and another way and another way and so
on, then you have like two to the 300 classical bits, which is as many particles as there
are in the universe.>>But there is a catch, although the qubits
can exist in any combination of states, when they are measured they must fall into one
of the basis states. And all the other information about the state before the measurement is
lost.>>So you don’t want generally to have as
the final result of your quantum computation something that is a very complicated super
positional state, because our cannot measure a super position. You can only measure one
of these basis states.>>Like down, down, up, up.>>Yeah. So what you want is to design the
logical operations that you need to get to the final computational result in such a way
that the final result is something you are able to measure, just a unique state.>>That is not trivial.>>That is not trivial. And it is essentially
… I am kind of stretching things, but I guess it is to some degree the reason why
quantum computers are not a replacement of classical computers.>>They are not.>>No, they are not. They are not universally
faster. They are only faster for special types of calculations where you can use the fact
that you have all these quantum super positions available to you at the same time, to do some
kind of computational parallelism. If you just want to watch a video in high definition
or browse the internet or write some documenting work, they are not going to give you any particular
improvement if you need to use a classical algorithm to get the result. So you should
not think of a quantum computer as something where every operation is faster. In fact,
every operation is probably going to be slower than in the computer you have at your desk.
But it is a computer where the number of operations required to arrive at the result is exponentially
small. So the improvement is not in the speed of the individual operation. It is in the
total number of operations you need to arrive at the result.
But that is only the case in particular types of calculations, particular algorithms. It
is not universally, which is why it is not a replacement of a classical computer.

100 Comments

  • ailan shangpliang

    Its like explaining Calculus to a stone age human. How on earth do these people even communicate with the sub atomic particles or whatever and get them to work?. mind bogging

  • thousand points of light

    a quantum bit ie a unit of light cannot be a one and zero at the same time.
    light is polarized, meaning it has its own magnetic field, a computer uses
    magnetic fields to name bits ones or zeros, a bit or unit cannot be both a
    one and a zero at the same time. what this is speaking of are fallen angels
    who are androgynous, ie both sexes, female body, male genitals.

  • soroush moallemi

    Could you please explain how we can detect these spins? It seems we need very sensitive sensors to determine the spin of the electron

  • Tyler Eury

    Doesnt that mean that they could create super defense systems against malware? Making hacking impossible with a normal computer? Idk

  • Stepler Velikiy

    Wow! I’m a bit shocked, because today i had a lecture in the university about the quantum superposition! Is it a coincidence that i have this video in my recommendation list?

  • Menya Savut

    is there any sane reason why you constantly move the camera, when you record Andrea? thumbs down for one of the worst cameraman.

  • katakis1

    He does a great job at explaining it. Picks his words with great care. Now I want to know which types of calculations would benefit from quantum computing.

  • Albert Souza

    Wtf did he just say!
    My brain just shattered in a million places.
    That's why he makes the big bucks
    Dammit I should have hung out with geeks! lol

  • The_DankSmith

    100% of people who watched the video understood everything but the other 100% of them didnt understand anything

    and thats called a superposition

  • Petr Antoš

    well, about information storage, the 2 power to 300 (why the hell the number is not also power of 2?), its simply say 40 bytes, so why the hell somebody wants to "enwow" somebody by explaining this and this shitty way? I understand its about some crazy paralel computations and very specific use cases, but as he perfectly described, even encoding such info into field of qubits is far far far far more complex, not to mention almost quantum-physically impossible decoding it – and nobody was talking about some basic functional parallel ""ALU"" (arithmetic-logical-unit) yet, so I am sure almost nobody knows how to use it all to do anything usefull now and I am almost (un)certainly sure that nobody will know how to do it during next few decades still )), .. yet, unsure

  • K U

    Thank you for Sharing this information; I have a better understanding in how these work and assurance that Skynet isnt taking over the world anytime soon!

  • Nhat

    the hardest part to understand is the both states at the same time, it makes us confuse but it's just the way it is, it's a property of quantum particles, it doesn't follow our physics rules

  • Ian Lonergan

    I’d like to find out how they actually program a quantum state computer, actually at a guess it would be beyond me to appreciate.

    I get to this stage each quantum bit (q-bit) has a capacity of being both 1 and 0 but as a probability of being both 1 and 0 at the same time. I could be wrong and sadly I’m probably will be but if one actually measures the actual position of a q-bit it should collapse like in a wave function of a split screen collapses when a photon is measured if it is going this way or that way. However before measuring the photons path the photon was “happy” going through both slits.

    Then they talk about having a series of q-bits which then increases the computational power by having these suppositions nested as each q-bit being so much of this and so much of that at the same time. My brain hurts……..like I got me a pair of gloves but I’ve put them on back to front only to find my little pinky don’t work like my thumb. Noooooooooo! I don’t care how safe it is I ain’t gonna drink any milk until I know wether I’m dead or alive or both or should I say a probability of being both dead and alive.

    Drink it Freddy it’s good for you! Gulp!

  • Narendra Roy

    Somebody please correct me if I am wrong. So.. Quantum computing according to him helps in problems that require too many parallel processes to arrive to answer but when it comes to problems which intrinsically demand sequential processing, it fails to provide us any better alternative than a normal classical bit system. Is it so? Speaking of the mechanics of its working, is it like we want to store as much variability as possible in the system which can be handled but with a sense that that should ultimately map back to our earlier idea of standardized states. So, we employ to some complicated linear/non-linear sum of probability values of spin states and then finally divide the output (values)space of that combination into some non-overlapping partitions which then serves as a well defined region for us to map as some classical bit state?

  • Adam Nowak

    This guy Andrea is doing a really good job explaining quantum computers. Most of the materials about it that i stumbled upon seem like the authors don't get the idea themselves. He sounds like he's having some deep understanding of the topic

  • M Gabrielle

    Thank God quantum computer cannot and will not replace my old pc. And a bit silly to rely on uncertainty of qubits, they can be both in a supper position I.e 1 and 0 at the same time, oh yes, who the heck do you think you are fooling? Why not say quantum computer only works on guesses, so the results would be just a fluke guess.

  • Arno Inen

    To simplify… one qubit can act like it is two bits (two switches) Two qubits like four. Three qubits like eight. Etc.

    That's the revolution.

  • AcidBot66

    I am still to see someone who explains how to use uncertainty and extract useful, predictable and reliable processing results.

  • M Gabrielle

    Also my wife is a quantum bitch, she be with me one minute and the next minute she is with someone, she is calculating who is better and it has taken her 10 years so far to arrive at any conclusion one way or another, so I am glad she is in a super position and neither here nor there or screwing me thinking its him, she is with me at the same time she is with him, really that is not an impossible state to be ! she is a real quantim bitch and can be everywhere at the same time. You cannot comprehend that only I know! .

  • john smith

    right this is my second "how quantum computers work" vid and I'm now finding myself not caring how the fecking things function

  • Kedarnath Rajeevkumar

    with one bit we can only represent a 0 or 1 ie; one digit but with one qubit we can represent 2 digits 1 and 0 similarly with 2 qubits we can represent 4 digits 00 01 10 11 @same time but with 2 bits we can represent only 1 and 0

  • M_Faraday

    Question: at 1:17 he says that if you put it exactly against the magnetic field it would stay there. Is this just Buridan's ass but with electrons?

  • Dennis Kean

    I have read up about this nonsense for years. Having a perfect score in all my years of math study I can honestly say that this is totally ridiculous. There is nothing and no advantage to be gained mathematically with switching from indeterminate to a defined state. Universities use this as a cash cow for getting funds and doing nothing more than talking. It is just like the environmentalists. "Give me lots of money and I will pacify you if you give me enough".

  • nishant jhala

    don’t need to wait for quantum computers to come as it’s not a replacement of classical computer. We can buy the existing classical computer as it’s not going to change our type of computer usage, we won’t regret buying the classical computer even if the quantum computer is launched next day. so chill.

  • Seudon Amous

    It would be a requirement of the virtual reality we are living in though. What we need now is a quantum computer in the VR world to help the Quantum Computer in the real world do its job.
    I am old and the glitches have become noticable…. It was a little traumatic

  • jassonsw

    I still haven't heard a decent explanation of quantum computing. I get qubits, I get entanglement but how the hell do you make that system perform any calculations? How do you manipulate the qubits so they do something useful? They never explain the guts of it properly, they always talk in analogies. "It's like this" or "It's like that". Don't tell me what it's LIKE tell me what it bloody well IS!

  • Victor Tong

    I watched this video couple times, there are couple things I still haven’t figure it out.

    If the results can only reveal at the end at once,

    So it is quantum memory to reveal the true state, once it is reveal it changes, but the state is fuzzy state , which also the best guess of something.

    And u mention it’s works out the same for all the lightbulbs or whatever have electron , which implies it can build with any materials.

    If so, then we can build it at home?

  • SYLperc

    I think i get it..

    A classical computer is faster for single computations. It does one thing at a time very quickly. And as every bit it has available increases, it can do another single computation per bit available – this is a linear increase.

    For quantum computing, it can do a single computation per bit available. However, when you add available bits, it can run in parrallel an exponential number of computations.

    The difference is noted when the computing power is used for finding results of a function with a changing variable. A classical computer must run each equation one bit (or 2) per t time, and if there are n bits, it will run at 2n bits per t time. Whereas with a quantum computer, it can run a function at one bit (or 2) per t time, and with n bits, 2^n bits per time.

    So for say trying to find the encryption password for any secure network, quantum computing will find it exponentially faster than classical computing.

  • Anuj Topiwala

    So technically using two qubit is equivalent of quad core computer for some problem and 64 qubit is 2^64 core computer.

  • Email Alpha

    I have not been able to find a video on quantum computers that went beyond the physics. Imagine looking for a tutorial on Javascript programming and every video you find talks about transistors. Does anyone know of a good video on how programming quantum computers actually works??

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